The present invention relates to an image display panel such as a liquid crystal display panel of active matrix type. More particularly, it relates to an image display panel which is made up of a substrate having an array of display elements thereon; each display element comprising a pixel electrode to display picture elements and a scan electrode to apply a display voltage, and display drive elements connected to the pixel electrode and scan electrode.
Recently display panels, especially liquid crystal display panels, have come into practical use for the display of television images, although the size is still somewhat limited. For the display panel to display images as large as those of conventional CRTs, it must have a larger area and higher display density than before. A display panel of sufficiently large area and high display density requires several hundred pixels in each of the horizontal and vertical directions, which is equivalent to tens of thousands of pixels on the entire surface of the panel. If these pixels are to be driven by their corresponding external circuits, the drive circuits would be very complex and expensive. This disadvantage is eliminated in the case of an active matrix system. According to this system, each pixel is provided with a simple display drive element which has been previously built into the substrate of the display panel. The display drive element built into the substrate needs to be of thinfilm type. The known display drive elements include metalinsulator-metal (MIM) elements, thin-film transistors (TFT), and a pair of diodes connected in parallel with opposite polarities. The display drive element based on diode pairs made of amorphous silicon thin film is considered to be promising on account of its low cost and uniform switching characteristics.
No matter what type of display drive element might be used, the decided disadvantage of the display panels of large area and high display density is that their yields are low. A display panel has tens of thousands of pixels on a substrate, as mentioned above. This means that a display panel needs hundreds of thousands of display drive elements in total because each display drive element of diode pair type needs two diodes, and a color display panel needs three times as many display drive elements as a monochrome display panel. It is very difficult to produce all of these elements completely free of defects. Although the defects which are liable to occur in the display drive elements differ from one type to another, defects occur less frequently than expected in the amorphous silicon layer in the case where the display drive elements are amorphous silicon diode pairs. Rather, defects occur frequently in the connections of pixel electrodes and scan electrodes. These defects lead to the short-circuit of display drive elements and, more frequently, the disconnection of such elements. FIG. 3 illustrates how defects occur in the conventional display panel that employs diode pairs as the display drive elements.
FIG. 3(a) is an enlarged plan view showing a part of a display in which a diode pair 31, 32 for one pixel is provided. FIGS. 3(b) and 3(c) are sectional views taken in the direction of arrows B--B and C--C respectively, in FIG. 3(a). FIG. 3(a) shows pixel electrode 10 corresponding to diode 31 and 32, and an adjacent pixel electrode 11, as well as a scan electrode 20 to apply a display voltage to a plurality of pixel electrodes arranged in the vertical direction in the figure. The pixel electrodes and the scan electrode 20 are formed on the entire surface of a colorless transparent glass substrate 1 from a transparent conductive metal oxide layer such as indium-tin oxide (ITO) having a thickness of between hundreds and thousands of angstroms by electron-beam evaporation or sputtering. On the metal oxide layer are formed diodes 31, 32 in the following manner. A light-shielding layer 30a, which is a 500-2000 .ANG. thick Cr layer is formed by sputtering. On the light-shielding layer 30a is grown an amorphous silicon layer 30 b of pin structure to a thickness of 0.5 to 1 .mu.m by plasma CVD method. On the amorphous silicon layer 30b is formed a Cr light-shielding layer 30c. The thus formed amorphous silicon layer of triple layer structure containing light-shielding layer undergoes reactive ion etching by photolithography method except those parts which become diodes 31, 32. Those parts which become diodes 31,32 remain unetched as shown in FIG. 3(a). The previously deposited ITO film undergoes chemical etching by photolithography to form the pixel electrode 10 and scan electrode 20 of desired pattern.
Then a 500-2000.ANG. thick insulation layer 61 of silicon nitride as a protective film is formed by CVD method on the entire surface. This layer undergoes etching by photolithography to form a pattern which covers the two diodes 31, 32 as shown in FIG. 3(a). At the same time, a window 61a is formed in the insulation layer on the top of each diode. This patterning is accomplished by gas etching. The light-shielding layer 30c under the window 61a prevents the amorphous silicon layer 30b from being etched by a reactive gas. The diodes 31, 32 are connected by an aluminum layer according to the usual practice. An aluminum layer having a thickness of thousands of angstroms is formed by sputtering such that it comes into conductive contact with the light-shielding layer 30c as the upper electrode layer under the window 61a. The aluminum layer is etched by photolithography to form the connection layers 41, 42 having a pattern as shown in FIG. 3(a). The connection layer 41 connects the top electrode of the diode 31 formed on the drive electrode 20 to the pixel electrode 10, and the connection layer 42 connects the top electrode of the diode 32 formed on the pixel electrode 10 to the scan electrode 20. Thus the connection layers 41, 42 establish the two diodes 31, 32 connected in parallel with opposite polarities between the pixel electrode 10 and scan electrode 20.
The display panel of conventional type formed as described above often suffers from disconnection on the substrate. The disconnection is attributable to the loss of the ITO layer at locations indicated by hatching, P and Q, in FIG. 3. P is between the connection layer 41 and the pixel electrode 10, and Q is between the connection layer 42 and the scan electrode 20. This defect occurs when the aluminum connection layer is formed by selective etching. This etching is achieved with phosphoric acid or hydrofluoric acid. This etching solution should not corrode the metal oxide layer, such as an ITO layer. In reality, however, corrosion sometimes does occur. To prevent the propagation of corrosion to other parts of the pixel electrode and scan electrode, a protective layer is formed under the aluminum layer. This protective layer is a Cr layer or Ti layer, about 1000 .ANG. thick, formed by sputtering. The aluminum layer, which is 5000-6000 .ANG. thick, is formed on the protective layer. The unnecessary part of the aluminum layer is removed by etching with phosphoric acid or hydrofluoric acid, and the unnecessary part of the Cr layer of Ti layer remaining after etching is removed by etching with a mixed aqueous solution of ceric ammonium nitrate and perchloric acid. The protective layer greatly reduces the occurrence of disconnection. A disadvantage of using the protective layer is that two additional steps are required for the deposition and etching of the metal layer, leading to an increased production cost. It may be possible to use the Cr layer or Ti layer alone as the connection layer, eliminating the aluminum layer. This alternative, however, has another disadvantage; that is, comparatively brittle Cr or Ti under thermal stress easily breaks at the step (about 2 .mu.m) between the diode and the pixel electrode or scan electrode as shown in FIGS. 3(b) and 3(c).